She needs them as a side effect of the Spring Maiden powers

Raven Branwen had had the powers of the Spring Maiden for a while ever since she killed the previous one. She believed having these powers would be a surefire defense against almost anything Salem's forces sent her way, given she could manipulate the elements around her and wipe out several Grimm in an instant.

However, it turned out there was a certain side effect to the Maiden powers nobody really knew about. For some reason, it seemed like ever since she became the Spring Maiden, she was completely incontinent, so the fearless leader of the Branwen Tribe needed to wear diapers 24/7, much to her embarrassment. She was only to stop herself from being mocked by her fellow bandits by threatening them with her powers... or threatening to have them empty out her diaper pail as punishment.

Raven was currently drinking some tea in her tent when she saw someone enter.